Lomax A
Department of Radiation Medicine, Paul Scherrer Institute, Villigen, Switzerland.
Phys Med Biol. 1999 Jan;44(1):185-205. doi: 10.1088/0031-9155/44/1/014.
The characteristic Bragg peak of protons or heavy ions provides a good localization of dose in three dimensions. Through their ability to deliver laterally and distally shaped homogenous fields, protons have been shown to be a precise and practical method for delivering highly conformal radiotherapy. However, in an analogous manner to intensity modulation for photons, protons can be used to construct dose distributions through the application of many individually inhomogeneous fields, but with the localization of dose in the Bragg peak providing the possibility of modulating intensity within each field in two or three dimensions. We describe four different methods of intensity modulation for protons and describe how these have been implemented in an existing proton planning system. As a preliminary evaluation of the efficacy of these methods, each has been applied to an example case using a variety of field combinations. Dose-volume histogram analysis of the resulting dose distributions shows that when large numbers of fields are used, all techniques exhibit both good target homogeneity and sparing of neighbouring critical structures, with little difference between the four techniques being discerned. As the number of fields is decreased, however, only a full 3D modulation of individual Bragg peaks can preserve both target coverage and sparing of normal tissues. We conclude that the 3D method provides the greatest flexibility for constructing conformal doses in challenging situations, but that when large numbers of beam ports are available, little advantage may be gained from the additional modulation of intensity in depth.
质子或重离子的特征布拉格峰在三维空间中能很好地定位剂量。通过其提供横向和远端形状均匀射野的能力，质子已被证明是一种精确且实用的高剂量适形放疗方法。然而，类似于光子的调强放疗，质子可通过应用许多单独的非均匀射野来构建剂量分布，但布拉格峰处的剂量定位使得在每个射野内进行二维或三维的强度调制成为可能。我们描述了四种不同的质子调强方法，并阐述了它们在现有的质子治疗计划系统中是如何实现的。作为对这些方法疗效的初步评估，每种方法都使用多种射野组合应用于一个示例病例。对所得剂量分布的剂量体积直方图分析表明，当使用大量射野时，所有技术都能实现良好的靶区均匀性并保护相邻的关键结构，这四种技术之间几乎没有差异。然而，当射野数量减少时，只有对单个布拉格峰进行完整的三维调制才能同时保证靶区覆盖和正常组织的保护。我们得出结论，三维方法在具有挑战性的情况下构建适形剂量时具有最大的灵活性，但当有大量射束端口可用时，深度方向上额外的强度调制可能不会带来太多优势。
